This invention relates to ink jet printing systems in general and to systems for maintaining satisfactory operation thereof in particular. Ink jet systems are generally known in this art and involve the use of electrically conductive ink compositions which are subjected to vibratory energy to create drops. The drops are then selectively charged for deflection by electrostatic plates onto a substrate to be marked. See, for example, U.S. Pat. No. 4,555,712. Drops which are not directed at the substrate return to an ink reservoir for reuse. As the system operates, ink is lost through printing and evaporation. This, as well as the effects of temperature and other system variations, causes the ink flow rate to vary, sometimes considerably. To sustain printing operations over a long period it is necessary to monitor and control the ink flow rate. Typically, control systems monitor the change in flow rate over a period of operation and, responsive to detection of changes, alter the ink composition or add make-up ink thereby to keep the flow rate substantially constant. A typical action is to periodically operate a valve permitting the addition of make-up ink to the ink reservoir to compensate for the loss of solvent from the ink.
One prior art approach to maintaining ink flow rate substantially constant is disclosed in the '712 patent, assigned to the present assignee. That patent is hereby incorporated by reference. The present invention is an improvement upon the '712 patent and reference thereto is made for additional details concerning an ink jet system in which the present invention may be utilized.
The device described in the '712 patent uses a simple scheme for determining operation of the make-up ink valve. If the flow time measurement reading is greater than the set point time the valve is actuated for a fixed time duration. This occurs once each system cycle until the flow time is less than the set point time. In some instances, an average flow time over several cycles is compared to the set point.
This type of flow control in which make-up ink is added or not based upon a comparison with the set point flow time can be referred to as a "go/no go" control. Such a method inherently causes oscillation in the flow time characteristic curve. For high evaporation rate inks the amplitude of oscillation can reach an unacceptable two seconds peak-to-peak. The problem can be further exaggerated if the operator does not pick optimum setup parameters for the system.
Oscillations in flow time characteristics are due to at least two sources. First, the total volume of ink in the system causes mixing time constant and process delays to occur. Therefore, corrective action occurs over a period of time as make-up ink propagates through the system. This insures that an over correction will occur before the corrective action is detected by the flow rate controller. Once corrective action stops, it does not resume until an under corrected condition is detected.
A second cause of oscillation is that even a small error in flow time results in initiation of a full ink addition cycle. Thus, the system acts like a high gain control system which is prone to oscillation.
It is desired to provide an improved method of controlling flow rate in an ink jet printer. Specifically, it is desired to provide a system in which the ink flow time may be maintained at a set value without significant deviation therefrom.
A further object of the invention is to provide a steady flow of make-up ink to compensate for evaporative losses without undesirable oscillations in the addition process.
A further object of the invention is to provide a system which can automatically accommodate a wide range of inks having different evaporative loss rates without the need for manual calibration by providing a self-adjustment capability which is not directly dependent upon operator skill.
Another object of the invention is to provide a system which can self-adjust as necessary and which can self-correct in the case of operator set-up error.